Arsenic (As) is one of the most hazardous metals present in the environment. It occurs mainly in the form of an arsenite (As(III)) or as an arsenate (As(V)). Arsenic(III) is generally more toxic and more mobile than arsenic(V). Long term exposure to arsenic in drinking water at concentrations significantly above 50 μg/L can cause serious health problems including cancers, melanosis, hyperkeratosis, restrictive lung disease, gangrene, hypertension and peripheral vascular disease. Elevated levels of arsenic in groundwater have been documented in many countries, including in Argentina and Chile in South America, several countries of North and Central America, and India, Bangladesh, Vietnam, Japan and China in Asia.
The WHO (World Health Organization) provisional guideline for the acceptable level of arsenic in drinking water is 10 μg/L. However, many countries, including Bangladesh and China have retained the earlier WHO guideline of 50 μg/L as their standard. In 2001, the Environmental Protection Agency in the U.S. published a new 10 μg/L standard for As in drinking water, requiring public water supplies to reduce As from 50 μg/L to below this limit.
Arsenic is released into the environment by natural activities such as volcanic action, erosion of rocks and forest fires. Thus, water in lakes and rivers can become contaminated with arsenic from natural sources. Arsenic is also released from soils and sediments into groundwater by geogenic processes. In addition, arsenic is present in the environment as a result of industrial activities, including mining and smelting operations, agricultural applications, and the use of industrial products and subsequent disposal of wastes containing arsenic. Due to its toxicity, arsenic has received major attention in the metallurgical industry, especially with respect to processes for the production of value metals such as copper, gold, zinc, cobalt and silver from their ores and concentrates. Despite this attention, there is still a need for processes to lower the concentration of arsenic in process solutions and waters, and especially processes that are friendly to the environment.
Arsenic has been removed from water using nanoscale magnetite, as disclosed by J. W. Farrell et al in Environmental Engineering Science, vol. 31, no. 7, 2014, page 1-10, using a column with a blend of magnetite and sand sandwiched between layers of sand. Reference is made to methods of removal of arsenic from water using a variety of materials, including goethite, hematite, magnetite, zero-valent iron and granular ferric oxide. The removal of arsenic from water with elevated concentrations of silica is disclosed by I. D. Dinkeiman, University of Nevada, Reno in Dissertation & Theses—Gradworks, 2008, reference 1461529 in which arsenic is removed using Kemira™ CFH-12 and CFH-18 as well as Bayoxide™ E33, all of which are stated to be granular ferric oxide. U.S. Pat. No. 7,615,199 of J. Poijarvi et al discloses a two-stage process for removal of arsenic from acidic liquid obtained from sulphuric acid leaching of a value metal from a source material in which solution is treated with a mixture of calcium carbonate and ferrous sulphate in a first stage and then a mixture of ferrous sulphate and calcium hydroxide in a second stage. In Treatment of Copper Smelting and Refining Wastes, Bureau of Mines, US Department of the interior RI 9522, 1994, D. K. Steele et al disclose the use of tributyl phosphate (TBP) in separation of arsenic and molybdenum from acidic copper solutions having a pH of 0-1.
There exists a need in the art for improved processes for the removal of arsenic from aqueous solutions containing arsenic, such as, for example, water contaminated with arsenic.